This invention relates to liquid drop printing systems of the type wherein drops from a continuous stream are electrostatically deflected to various flight paths. More specifically, this invention relates to improved method and apparatus for electrostatically deflecting charged drops in such systems.
U.S. Pat. No. 3,596,275 to Sweet describes a printing, marking, recording or imaging system of the present type. A continuous stream of drops is formed from a column of liquid emitted from a nozzle under pressure. At the point of drop formation from the columns, the drops are charged as they pass through a tunnel electrode. Downstream, an electrostatic field created between a pair of planar electrodes or plates on opposite sides of the drop stream deflect charged drops proportionally to their charge. The drops are spread out in a straight line on a stationary target at which they are directed. However, the nozzle and target usually move generally normal to the electrostatic deflection field. Consequently, the line pattern created by the deflection process is distorted by the relative movement.
Heretofore, distortion due to relative motion has been compensated for by tilting the deflection plates. Also, it is known to shift in time the charging of drops intended for certain positions within a line of drops thereby offsetting the tilt electrically. Mechanical tilting of the deflection plates presents packaging and maintenance difficulties in a multiple nozzle printing system. An exception are binary deflection systems wherein tilted deflection plates permit the spacing between nozzles to be increased thereby improving the apparent nozzle packing density. Prior to this invention, it has been unknown to tilt the deflection electrodes in a system creating a full scan line by stitching together segments of the scan line created by each nozzle. Electrical tilting of deflected drops is disadvantageous in that the drop utilization efficiency falls down. This means the printing rate is slower. In addition, special buffer memory is necessary to handle the electrical correction signals that take the tilt out of a scan line. Again, the electrical tilt correction becomes very complicated when employed in a multiple nozzle system.